This invention relates to the field of making small capsules, or microcapsules, having a core material encased within a shell or wall material. In this specification, the terms "shell" and "wall" are used with identical meanings.
A microcapsule has a diameter of the order of about 5-5000 microns. Microcapsules have many applications, such as in the manufacture of pharmaceuticals, pesticides, paints, adhesives, and many other chemical products. Microcapsules are especially useful where it is desired to provide a controlled release of the substance being encapsulated. The product known as "carbonless paper" is made by providing a liquid dye in microcapsules, so that the dye is released when pressure ruptures the capsule walls.
Examples of processes for forming microcapsules are given in Vandegaer, "Microencapsulation Processes and Applications", Plenum Press, New York, 1974, M. Gutcho, "Microcapsules and other Capsules", Chemical Technology Review, No. 135, Noyles Data Service, Park Ridge, N.J. 1979, and the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition (1981), volume 15. Other references disclosing processes for forming microcapsules include U.S. Pat. Nos. 3,943,063, 3,460,972, 4,001,140, and 4,087,376. All of the above-mentioned publications and patents are incorporated by reference herein.
The above-mentioned references describe several liquid-phase methods of encapsulation. These methods include coacervation, thermal coacervation, complex coacervation, interfacial polymerization, and others. In the process of coacervation, the core and shell materials are mixed together in a liquid medium. When the core and shell materials have been agitated for a sufficient period of time, portions of the core material become coated with shell material, thus forming capsules within the liquid medium. The size of these capsules is controlled by the speed and design of the mixing element within the vessel. The thickness of the shell material is adjusted by a further chemical treatment process.
The coacervation process described above has many disadvantages. It is difficult to achieve precise control of the size of the microcapsules. Inadequate agitation of the mixture frequently produces capsules which are too large, often beyond the size range suitable for the desired application. In the coacervation process, it is also difficult to adjust the thickness of the shell of the capsules. A thicker shell is often essential to enhance the shear and impact resistance of the capsule, and to enable the capsule to withstand high temperatures.
In addition to the disadvantages discussed above, the coacervation process is very time-consuming. The core and shell materials must be stirred for a long period of time, on the order of several hours, before usable capsules are produced. The time required to form the capsules adds significantly to the cost of their manufacture.
Conventional liquid-phase methods of making microcapsules, such as the coacervation process mentioned above, often produce unsatisfactory quantities of microencapsulated products. Moreover, it often happens that the core material is soluble in the liquid medium, in which case such materials dissolve in the liquid medium long before encapsulation can occur.
There is presently a great demand for microcapsules which can be inexpensively manufactured, and which are suitable for various industrial applications.
Microcapsules used in industry must exhibit the following properties:
1. The capsules must be capable of withstanding large shear forces, or other stressful conditions, when the capsules are added to a host material. Suitable host materials could be paints, plastics, foam products, building materials, paper products and others. Each host material requires varying conditions of heat and stress to produce the final product, and the capsules must have suitable physical properties to enable the capsules to be used during the manufacture of the final product.
2. Capsules used in industry must generally be very small. Microcapsules made by conventional liquid-phase methods of encapsulation, and by other methods, usually have an unacceptably wide size distribution, and are often too large for use in industrial processing.
3. Capsules used in industry should be produced in a continuous process, so that the capsules are available in large quantities, and at relatively low cost.
The present invention provides a process and apparatus for making microcapsules which have the properties described above. The process of the present invention can produce microcapsules in a small fraction of the time required by conventional methods. The present invention also permits the accurate adjustment of the size of the capsules and the thickness of their shells.